Cast an unused variable to void during NDEBUG builds

Summary:
the only usage of `remaining` here is in an assert and thus clang13
errors out on NDEBUG builds. Give it a trivial "usage" here.

Reviewed By: smeenai

Differential Revision: D29719883

fbshipit-source-id: 8141c3ee8ed1b852ea04ec03954c8fe1ac83c12b

folly/io/IOBuf.cpp

@@ -954,6 +954,7 @@ void IOBuf::coalesceAndReallocate(
     current = current->next_;
   } while (current != end);
   assert(remaining == 0);
+  (void)remaining;
 
   // Point at the new buffer
   decrementRefcount();